This invention relates to digital switching network and, more particularly, to call conferencing in a multi-line telephone systems which provide an interface between large key telephones and a multiplicity of analog and digital lines.
Multi-line telephone key stations, sometimes referred to as "trader turrets" are widely used in rapid communication networks such as trading operations in banks, brokerage houses, and other financial institutions. Telephones of this type provide direct access to a large number of telephone lines with a line being selected by the depression of a single key on the key station. A large trading room can include many telephone key stations for simultaneously completing many transactions each involving many millions of dollars;. Trader turrets generally include at least thirty line keys and often have several hundred. Trader turrets are normally used in networks where the number of lines is greater than the number of telephones whereas, with normal telephone key stations, the number of telephones exceeds the number of lines.
A typical trading transaction involves a rapid series of short telephone conversations to, for example, locate traders having the desired security for sale at the best price. A typical call lasts for few seconds and there may be many calls per minute during the course of a single transaction. It is important for a trader to have immediate access to outside lines irrespective of system load conditions. It is also important that many terminals and users could be connected to each other simultaneously in a conference mode in a highly distributed switching system.
Each telephone key station may have access to other telephone key stations in the system within a trading room and to a large number of outside lines and private lines. Each telephone user may be connected to several lines or terminals at the same time in a conferencing mode.
In prior art switching systems, conferencing was accomplished by connecting three or more parties to a dedicated conference bridge for the system. With such arrangements there is a maximum number of parties that can participate in a conference. There is also a maximum number of conferences that can exist at any one time.
A dedicated switching element to accomplish the conferencing function and to serve as a conference bridge was provided. In most of the digital switches of the prior art voice data was companded. As a result, voice data had to be linearized at the conference bridge before the voice data from different sources could be summed at the conference bridge. The linearized voice data would be summed at the conference bridge but the sum had to be returned to the companded state. This approach tends to make the system rigid and inflexible to system changes.